“Click” and “pop” are terms used to describe unwanted audio-band transient signals that are heard in a headphone or a speaker when an audio amplifier is powered down.
In portable audio applications power consumption is a key issue, which means that circuit components, such as audio amplifiers, are often disabled or powered down when not required. This can lead to unwanted audio-band transient signals being produced each time an audio amplifier is powered down or placed in a sleep or hibernation mode. Similar problems can also arise in other non-portable applications.
Click and pop problems are particularly problematic in single supply amplifiers that have to charge to a certain defined voltage during power-up, which then has to be discharged during power-down.
FIG. 1 shows a known audio amplifier circuit 1 for driving a load 2, for example a headphone or a speaker, coupled to an output terminal 3. An output amplifier 5 receives an audio signal at a first input terminal 7 from an audio source, such as a mixer 9. It will be appreciated that the mixer 9 receives an audio signal from a DAC (not shown) or other signal source. The amplifier 5 also receives a reference voltage VMID at a second input terminal 11. In order for the output signal of the amplifier to achieve maximum swing, either side of its quiescent voltage, this quiescent voltage is set midway between the supply voltages VDD and ground (GND). The quiescent voltage is set by an applied reference voltage VMID equal to VDD/2.
The reference voltage VMID is produced by a reference voltage generator circuit 13. As will be described in greater detail below, a transient signal may be produced when the output amplifier 5 is powered down, thereby causing an unwanted “pop” being transmitted to the headphone or speaker.
It is noted that control logic 10 is provided for controlling the operation of the amplifier circuit 1 during various modes of operation. For example, the control logic 10 provides a control signal S1 for controlling the reference generator circuit 13, a control signal S2 for controlling the output amplifier 5 (for example when performing a mute operation), and a control signal S3 for controlling a buffer circuit 14. The buffer circuit 14 buffers the reference voltage VMID received from the reference voltage generator circuit 13. It is noted that the buffer circuit is not essential to the operation of the amplifier circuit.
FIG. 2 illustrates an example of a power-down sequence for an audio amplifier according to the prior art. The first step, step 201, involves muting the output amplifier 5 using the control signal S2 of the control logic 10. In the mute state the output is unaffected by the input signal, for example by interrupting the signal path using a switch. Next, circuit components upstream of the output amplifier 5 are disabled, for example the mixer 9, DAC (not shown), etc., step 203. After the upstream circuitry has been disabled, the reference voltage generator circuit 13 that produces the reference voltage VMID is then disabled, step 205. This is performed, for example, by opening the switch 131 of FIG. 1 using control signal S1 from the control logic 10.
There is a delay while the reference voltage VMID falls to 0 v, step 207. This delay can take approximately 1 second depending on the total capacitive load. Once the reference voltage VMID has fallen to 0 v, the output amplifier 5 is then disabled or powered down, step 209.
When performing a power-down sequence such as that described above, a “pop” can be heard when the reference voltage VMID begins to discharge to ground, as will be described in further detail below.
FIG. 3 shows the reference voltage generator circuit 13 for producing the reference voltage VMID. The reference voltage VMID can be produced using a potential divider circuit, for example, that comprises resistive elements 137 and 139. If the voltage level of the reference voltage is chosen to be VDD/2, then the resistive elements 137 and 139 will have equal values. It will be appreciated that the resistive elements 137 and 139 would have different values if a different reference voltage was required. A decoupling capacitor 135 is connected across resistive element 139. It is noted that, in the case of an integrated circuit arrangement, the decoupling capacitor 135 may be provided off-chip, if desired, and is used to decouple the VMID node 133. A switch 131 is provided for enabling and disabling the reference voltage generator circuit 13, under control of the control signal S1.
FIG. 4 shows the reference voltage VMID at node 133 during power-down of the amplifier circuit 1. When the reference voltage generator circuit 13 is switched off at tOFF, for example by opening switch 131, the capacitor 135 is discharged through resistor 139. This results in a slope discontinuity or rapid deviation in the reference voltage VMID at tOFF. As the decoupling capacitor 135 continues to discharge, the fall in voltage level of the reference voltage VMID becomes more gradual until the decoupling capacitor 135 is fully discharged. This slope discontinuity of the reference voltage VMID at tOFF produces audible signal components that propagate through capacitor 15 and onto the load, and thus also causes an audible pop.
One method of avoiding these slope discontinuities would be to increase the value of resistor 139. However, since the total time taken to discharge the capacitor 135 depends on the value of resistor 139, an increased value of resistor 139 would lead to an unacceptably long discharge time (several seconds), whereas the discharge time is desired to be a few hundred milliseconds.
It is therefore an aim of the present invention to provide an amplifier power-down apparatus and method for reducing unwanted signals in an audio circuit.